\section{RGB color space}
\label{app:One}
The digital images often coded into a $\langle red,green,blue \rangle$ trichromatic space since this is that way that the human eye perceive the color and consequently the method used by our screens to reproduce it. 
A image in the \emph{RGB} color space is composed of three channels and each channel is separately represented in an image~\cite{carron1995segmentations}. All the calculus applied in this paper can be transposed to \emph{RGB} images by applying separately to each channel.\\
Also, the difference image $\Delta I$ can be computed like that~:
\[
\Delta I(x,y,t)=\dfrac{|R(x,y,t) - R_{ref}(x,y,t)| + |G(x,y,t) - G_{ref}(x,y,t)| + |B(x,y,t) - B_{ref}(x,y,t)|}{3},
\forall (x,y)
\]